Synovial fibroblasts destroy articular cartilage and bone in rheumatoid arthritis, but the mechanism of fibroblast transformation remains elusive. Since gain-of-function mutations of BRAF can transform fibroblasts, we examined BRAF in rheumatoid synovial fibroblasts. The strong gain-of-function mutation, V600R, of BRAF found in melanomas and other cancers was identified in first passage synovial fibroblasts from two of nine RA patients and confirmed by restriction site mapping. BRAF-specific siRNA inhibited proliferation of synovial fibroblasts with V600R mutations. A BRAF aberrant splice variant with an intact kinase domain and partial loss of the N-terminal autoinhibitory domain was identified in fibroblasts from an additional patient, and fibroblast proliferation was inhibited by BRAF-specific siRNA. Our finding is the first to establish mechanisms for fibroblast transformation responsible for destruction of articular cartilage and bone in rheumatoid arthritis and establishes a new target for therapeutic intervention.
Rheumatoid arthritis (RA) is a chronic inflammatory disease that occurs in 1% of the population and is characterized by progressive erosive arthritis of multiple joints associated with increased mortality. Although the inflammatory reaction contains numerous cell types, synovial fibroblasts have been identified as the cell responsible for invasion and destruction of cartilage and bone (Muller-Ladner, U., et al. (1996) Am J Pathol, 149, 1607-1615; Pap, T., et al. (2000) Arthritis Res, 2, 361-367; Buckley, C. D., et al. (2001) Trends Immunol, 22, 199-204; Muller-Ladner, U., et al. (2005) Nat Clin Pract Rheumatol, 1, 102-110; Pap, T., et al. (2005) Ann Rheum Dis, 64 Suppl 4, iv52-54; Karouzakis, E., et al. (2006) Immunol Lett, 106, 8-13; Huber, L. C., et al. (2006) Rheumatology (Oxford), 45, 669-675). Rheumatoid synovial fibroblasts show evidence of transformation indicated by excessive proliferation, loss of contact inhibition, and increased migration (Pap, T., et al. (2000) Arthritis Res, 2, 361-367; Karouzakis, E., et al. (2006) Immunol Lett, 106, 8-13; Mercer, K., et al. (2005) Cancer Res, 65, 11493-11500). Transformation of RA synovial fibroblasts has also been demonstrated in an animal model of RA employing xenograft implants of RA synovium as evidenced by metastasis of implanted synovial fibroblasts with localization and binding to cartilage (Lefevre, S., et al. (2009) Nat Med, 15, 1414-1420). The mechanism of RA synovial fibroblast transformation has not been identified, but it is critical for the rational design of therapies to prevent joint destruction. In spite of evidence for neoplastic transformation of RA synovial fibroblasts, oncogenes potentially responsible for transformation have not been identified. Since gain-of-function mutations in the BRAF oncogene have been shown to transform embryonic fibroblasts but not several other somatic cell types, we examined rheumatoid synovial fibroblasts for the presence of BRAF mutations (Mercer, K., et al. (2005) Cancer Res, 65, 11493-11500).